Conventionally known gas separation methods include, for example, (i) chemical absorption method, (ii) cryogenic separation method, and (iii) adsorption method. Although these methods have been widely used, each has merits and demerits and therefore, has been employed in fields for which the method is suited.
The chemical absorption method (i) has been used for the removal of hydrogen sulfide or carbon dioxide gas and has also been put to trial use for the desulfurization of exhaust gases. However, this method is defective in that in the case of using an organic compound as an absorbent, there are problems in treatment of waste fluid, treatment of harmful substances resulting from decomposition of the absorbent, etc., and that in the case where an acidic gas is treated using a hot aqueous alkali solution as an absorbent, the consumption of heat energy is large.
The cryogenic separation method (ii) has been used, for example, for the separation of air and the separation of hydrocarbon gases such as natural gas. However, this method is disadvantageous in that a large-sized freezing equipment is required and this method is costly. Therefore, practical use of the cryogenic separation method is limited to applications in which separation by the other methods is difficult.
The adsorption method (iii) has been extensively used because it is simple and the unit used therefor can have a size ranging from a small to a relatively large one. Known types of units for this method include fixed bed type and fluidized bed type.
In adsorption, the amount of a gas adsorbed onto an adsorbent becomes larger with increasing pressure and decreasing temperature, and becomes smaller with reducing pressure and increasing temperature. The adsorption method utilizes this phenomenon in conducting the adsorption step where a gas is adsorbed onto an adsorbent and the desorption step where the adsorbed gas is desorbed from the adsorbent. Adsorption separation units of the fixed bed type can utilize the above phenomenon by being provided with a means for changing pressure and temperature. However, in the case of adsorption separation units of the conventional fluidized bed type in which fluidized adsorbent particles circulate in the unit, a pressure difference is rarely utilized in the adsorption-desorption operation although a slight pressure is applied as a driving force for circulating adsorbent particles, from the standpoints of smooth migration of adsorbent particles between the desorption part and the adsorption part. For these reasons, the adsorption-desorption operation in the conventional units of the fluidized bed type utilizes a temperature difference only. In the case of adsorption separation units of the fixed bed type, since a larger bed height results in an increased pressure loss, the area of the adsorbent bed should be increased, or the whole unit should be enlarged, in order to heighten treating capacity. However, the possible unit size is limited. Furthermore, size increase of switch valves is also limited.
With a recent increase in the amount of chemical products produced in a single plant in the chemical industry or the like, the amount of gases to be treated by gas separation has become large. Therefore, there is a need of developing an adsorption method capable of coping with such recent large amount gas separation treatment.
Studies have recently been made of the separation and fixation of carbon dioxide gas present in fossil fuel-combustion gases as one means for preventing the earth from warming up due to carbon dioxide gas, and it has been proposed to use an adsorption unit of the fixed bed type for the carbon dioxide gas separation. However, in such carbon dioxide gas separations from combustion gases including exhaust gases discharged from thermal power stations, an enormous amount of gas should be treated and treating operation is required to be conducted continuously using an exceedingly large-sized unit. Treatment of such a large amount of gas using a fixed bed type unit is, therefore, difficult because of the limitations on adsorbent bed thickness and on size increase of switch valves as described above.